Project Summary Parkinson's disease (PD) is a chronic progressive neurologic disease that is the most common degenerative cause of impaired movement. It is estimated to affect 1 million Americans and its prevalence is predicted to double in developed nations by 2030 as the average age of these populations increases. PD has been the prototypic neurodegenerative disorder for which effective medical and surgical therapies exist to alleviate symptoms. However these treatments neither protect nor restore neural systems and there is a progressive deterioration in the quality of life. There is therefore a compelling need to develop therapies for this disease that prevent ongoing degeneration. The development of such therapies depends on a better understanding of the molecular mechanisms underlying the disease process such that critical mediators can be targeted. Since the discovery that mutations in the gene for ?-synuclein can cause PD, it has become a principal molecule of interest in the pathogenesis. There is now a growing consensus that, at a cellular level, ?-synuclein pathology first appears in axons. Furthermore, in PD it is clear that substantial damage has occurred to the axon projections by the time of diagnosis. The fact that ?-synuclein pathology first appears in axons does not necessarily mean that that they are the site of the earliest disease-related events at the molecular level. There is a substantial body of evidence that the earliest molecular events may occur within the nucleus. Where ?- synuclein first acts at a molecular level to initiate the pathological events underlying PD has not previously been addressed in a living mammalian system. The purpose of this proposal therefore is to address this fundamentally important question in vivo in mice using an AAV 2/7 h-?-synuclein(WT) model to deliver differentially targeted forms of ?-synuclein. We hypothesize that ?-synuclein acts first within the nucleus to initiate the pathologic events that underlie the onset of PD. We will test this hypothesis in two Specific Aims in which we will examine the differential effects of targeted forms of synuclein on the number and morphology of axons in the medial forebrain bundle. Our proposal is unique not only for being the first to examine the initial site of action of ?-synuclein in vivo, but also for using novel methodologies, developed in our lab, for monitoring axonal populations as the most sensitive indicators of early disease-related events. Our results will provide a much-needed fundamental advance in our understanding of how synuclein causes this disease.